The invention disclosed in the parent application, application Ser. No. 07/532,323, involves a composite transducer assembly with an input transducer and an output transducer. The input transducer is oriented facing out from the ear and toward the source of noise to be cancelled. The output transducer, on the other hand, is oriented to face in the opposite direction toward the ear canal of the user.
If ideal transducers were available with flat amplitude versus frequency response and with identical phase response versus each other, which instantaneously converted sound waves into electrical signals and visa versa, and if the source signal arrived at the ear and at the input transducer at exactly the same time, and if the circuitry to invert the input electrical signal from the input transducer did not introduce a delay or the ballistics of the transducers did not introduce a delay, all acoustic energy entering the ear canal would be canceled by the opposing wave output from the output transducer.
Transducers, however, are not ideal. Specifically, input transducers such as small electret microphones have a different phase response from output transducers such as small headphone type speakers. Similarly, output transducers such as a small headphone type speakers have a characteristic roll-off in output amplitude versus frequency of the input electrical signal. This roll-off occurs at low frequencies.
More specifically, electret microphones have a reasonably flat amplitude versus frequency response over a wide range of the audio frequency spectrum (e.g. .+-.3 Db 20 Hz-16 KHz). Due to the way an electret microphone is made, several problems exist in making it the ideal input device for use with an output transducer such as a small headphone type speaker.
The two most important differences between an electret microphone and a speaker are their different phase and amplitude versus frequency responses. An electret microphone element is basically a variable capacitor with a FET semiconductor to convert the changing capacitance into an electrical signal. As sound pressure is applied to the input diaphragm of the electret microphone, it changes capacitance in relation to the varying pressure. This capacitance variation is converted into a corresponding electrical signal by the FET semiconductor. The FET requires a DC voltage to operate. The output of the FET is then presented to the input of a preamplifier for further amplification to raise the signal to a usable level. The phase of the signal output by the FET may not be the same as the phase of the signal that would be output by an output transducer such as a small headphone type speaker. This different phase relationships of the input transducer and the output transducer produce an error in the desired exact 180.degree. phase shift desired between the input and output transducers. This error then does not allow complete cancellation of the source signal that reaches the ear canal.
Another problem that causes a phase shift difference between the input transducer and the output transducer is the use of a coupling capacitor between the electret microphone and its preamplifier. With current low cost semiconductors there is a maximum supply voltage that can be applied to the integrated circuit (IC) preamplifier. Typically this is .+-.18VDC maximum.
This maximum supply voltage also dictates the maximum output signal that can be generated by the IC. With a supply voltage of .+-.18VDC, the maximum signal swing that could be generated would be 36 volts. To amplify the signal from the input transducer to a usable level, amplification factors or gains of 500 to 1000 are needed. Since the electret microphone requires a DC voltage in the range of 2 to 10 volts to operate, the preamplifier IC would need to produce an output signal of 1000 to 2000 volts. This is of course not possible, so the DC supply voltage for the electret microphone can not be presented to the preamplifier for amplification. A capacitor is therefore used to block the DC voltage from the input to the preamplifier. This capacitor introduces a phase shift in the signal, thereby adding an error between the input transducer and the output transducer phase coherence. Some ICs can be offset by a given amount which corrects this problem, but the phase and the frequency response of the preamplifier IC working at the required gains is then altered resulting in another source of phase error. If an IC was capable of offsetting the supply voltage to the input transducer and amplifying the signal to the required level with flat amplitude and phase versus frequency response, it would be the ideal device for a preamplifier.
The output transducer also has problems that introduce error in the desired processed signal output. A output transducer such as a speaker does not have a flat amplitude versus frequency response over the same audio spectrum as the input transducer. This results in an error in the amplitude summation of the original source signal and the signal output by the output transducer. If the signals are not the exact same amplitude and 180.degree. out of phase, complete cancellation is not achieved. Output transducers such as small headphone type speakers roll-off at the lower frequencies. This does not allow complete cancellation of all frequencies. The low frequency response of the output transducer can be increased with the use of an enclosure to the back side of the output transducer much like a stereo loudspeaker uses to extend its low frequency response. This, however, does not lend itself to producing the composite transducer. The enclosure has to have a certain internal volume of air and would be bulky and heavy. The intent of the composite transducer is to be as small and light weight as possible. It is possible, however, for an enclosure to be put on the composite transducer which extends the low frequency response using materials such as plastic for weight reasons. If an enclosure is used to extend the response, it is still necessary for the original source signal to reach the input transducer at the same time as it reaches the ear canal.
Accordingly, when the invention of the parent application is realized using real input and output transducers, the less than ideal characteristics of the transducers affect the noise cancellation as the frequency of the input noise changes. If the gain and phase of the amplifier amplifying the output from the input transducer is changed so that the low frequency noise is cancelled, an incorrect electrical signal will then drive the speaker for high frequency noise. The low frequency components of the noise would be better cancelled but the high frequency components would not.
Accordingly, when non ideal transducers are used in the composite transducer assembly of the present invention, residual uncancelled noise is left in the ear canal.